Adding Random Noise to Rendered Images
I prefer the term Camera "Sensor noise" in this case. ISO (or gain) is just one of the many factors that might contribute to noise in images generated on a camera, and will be very different on different cameras.

Now on to your question.

Use a texture node with a noise texture in the compositor and mix that with your rendered image using a multiply node. That would give you control over the type of noise and its size, brightness and contrast. To animate the noise pattern you can animate the distortion values and/or the offset coordinates.



Sadly, Blender's compositor does not have a way to do buffered playback in real time, and you would need such a thing to be able to see the noise patterns moving from frame to frame and get a seamless integration. To do this in a more sophisticated and precise way you should also consider using other compositing apps like Fusion or Natron, those have built in tools to create different kinds of noise and to create random seeds for it.